msm: vidc_3x: Ensure validity of shared Q indices

Video driver and firmware communicates over shared queue.
The queue header has the indices which synchronizes the read
and write between the driver and firmware modules.
This change ensures that the indices are within the valid
range before accessing them.

CRs-fixed: 2345481
Change-Id: I8b49383920c86d09c9456b9790015f7473e87d03
Signed-off-by: Vikash Garodia <vgarodia@codeaurora.org>